A failed hunt for an exotic strangeonium reveals two strange particle structures

The particle zoo just got even stranger. Physicists searching for an elusive exotic particle instead found evidence of two unexpected structures that do not fit neatly into the familiar picture of quarks bound together in pairs. “We are in a new era here, similar to 70-odd years ago. First, a zoo of hadrons was discovered. Now, we’re facing a zoo of so-called exotic states,” Frank Nerling, one of the researchers and a scientist from Germany’s GSI Helmholtz Centre for Heavy Ion Research, said. The findings from the US Department of Energy’s Thomas Jefferson National Accelerator Facility could give researchers new clues about how the strong nuclear force—the force that holds the cores of atoms together—builds some of nature’s most interesting particles. A search that found something else The mystery begins with XYZ states, a growing collection of short-lived particles that have appeared in experiments since the early 2000s. The problem is that many of them do not behave as expected if they are simply a quark and an antiquark, as the traditional quark model suggests. One particularly puzzling example is Y(2175), a possible strangeonium particle with a mass of about 2.16 GeV. It was first reported in 2006 by the BaBar experiment at the US Department of Energy’s SLAC National Accelerator Laboratory. Later experiments at the Beijing Spectrometer and Belle in Japan supported its existence, but all of those observations came from collisions between electrons and positrons. This left a major question—would Y(2175) also appear when produced in a completely different way? Researchers with the GlueX Collaboration at Jefferson Lab set out to find out. Instead, their experiment produced evidence of two structures nearby: Y(2240), with a mass of about 2.24 GeV, and X(1830), with a mass of roughly 1.82 GeV. “We went searching for a confirmed XYZ candidate with a photon beam but instead found two other structures. It’s new information,” Malte Albrecht, one of the researchers and a staff scientist at Jefferson Lab, said. Turning electrons into a particle-hunting beam The GlueX experiment is built to investigate exotic mesons, particularly particles in which gluons—the particles that carry the strong nuclear force—may play a direct role. Researchers used Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF) to produce the experiment’s unusual probe. CEBAF sends electrons toward an ultrathin diamond wafer, which converts them into a high-energy photon beam with parallel spins. Those photons then slammed into protons inside a liquid hydrogen target, producing showers of secondary particles. A large spectrometer recorded the resulting debris, allowing researchers to reconstruct what particles had briefly existed inside the collision. The scale of the experiment is enormous. Millions of photons strike the target every second, generating enough data to fill an average laptop hard drive within only a few minutes. “No other experiment has a facility with a photon beam of this intensity at the energy we have available. This truly is a unique setup,” Albrecht said. The researchers sifted through this huge dataset looking specifically for evidence of Y(2175). It did not appear where they expected. Instead, two structures emerged at different masses. “One of the interesting things about this result is that we didn’t observe Y(2175) at the place we were searching. We found something new using a completely different physics process, and that’s really intriguing. But now that these have been observed, that doesn’t mean we’re done,” Albrecht said. The evidence for Y(2240) is particularly strong, reaching five sigma (5σ) significance, or about 99.9994% confidence. X(1830), meanwhile, reached 3σ significance, corresponding to about 99.7% confidence. The difference matters: the Y(2240) signal meets the commonly used five-sigma standard for a particle-physics observation, while the weaker X(1830) signal will require further evidence to establish its nature. Two new clues, but no final answers Finding the structures is only the beginning. Scientists still do not know what Y(2240) and X(1830) actually are. They could involve unusual arrangements of quarks, gluons, or combinations of already known composite particles. “Excited gluonic fields are what could be in these mesons where you have more than just the quark-antiquark pair. That’s one of the investigations, to try to understand whether there is a gluonic contribution to the structure we see,” Justin Stevens, a physics professor and a representative from GlueX. The new measurements give theorists fresh targets for testing competing explanations of exotic matter. They also place an upper limit on how often Y(2175) can be produced through photon collisions, helping researchers design future experiments and better understand how quarks and gluons behave inside matter. “The next step is to figure out which exotic quark configurations nature might have realized here. Theorists may come to further conclusions and identify measurements that could help pin down the real nature of these particular states,” Nerling added. With much more GlueX data still waiting to be analyzed, researchers expect the particle zoo to become even more crowded—and, potentially, a little easier to understand. The work adds to a broader effort to test the Standard Model of particle physics and understand whether the familiar framework fully explains the particles found in increasingly precise experiments.Get the latest in engineering, tech, space & science - delivered daily to your inbox.Rupendra Brahambhatt is an experienced writer, researcher, journalist, and filmmaker. With a B.Sc (Hons.) in Science and PGJMC in Mass Communications, he has been actively working with some of the most innovative brands, news agencies, digital magazines, documentary filmmakers, and nonprofits from different parts of the globe. As an author, he works with a vision to bring forward the right information and encourage a constructive mindset among the masses.